forget to do　忘記要去做某事。

　　forget doing　忘記做過某事。

　　The light in the office is stil on. He forgot to turn it off.

　　辦公室的燈還在亮著，它忘記關了。

　　He forgot turning the light off.

　　他忘記他已經關了燈了。

　　Dont forget to come tomorrow.

　　別忘了明天來。

　　典型例題

　　---- The light in the office is still on.

　　---- Oh，I forgot___.

　　A. turning it off　 B. turn it off　 C. to turn it off　 D. having turned it off

　　答案：C。由the light is still on 可知燈亮著，即關燈的動作沒有發生，因此用forget to do sth.

　　而forget doing sth表示燈已經關上了，而自己忘記了這一事實。此處不符合題意。